Molybdenum (Mo) metal is used for various specialty applications which require its unusual properties. The melting point of molybdenum is 2630.degree. C., which is over 1000.degree. C. higher than iron, thereby permitting molybdenum to be used for furnace components, rocket nozzles and in other high-temperature applications where most metals would melt or fail. Molybdenum also possesses resistance to corrosion by mineral acids when exposed to such acids under nonoxidizing conditions.
However, the high melting point and poor ductility of molybdenum requires that special manufacturing techniques be employed when producing molybdenum metal and articles therefrom. Molybdenum is usually manufactured as a powder. The molybdenum powder may be compacted into a bar by using conventional powder-metallurgy techniques, and the resulting bar then sintered and densified by electric currents or in a hydrogen-atmosphere muffle furnace. The use of hydrogen is required to eliminate any oxygen because even trace amounts of oxygen in molybdenum will adversely effect the ductility of molybdenum. In some molybdenum production processes, an ingot of molybdenum is obtained by arc-casting using consumable electrode melting under a vacuum.
The mechanical properties of an article fabricated from molybdenum are acceptable so long as the metal is shaped or worked below its recrystallization temperature, thus avoiding recrystallization and grain growth. When recrystallization has occurred, molybdenum has a tendency to become brittle at relatively low temperatures, e.g., near room temperature and below. Recrystallization becomes difficult to avoid when the process for manufacturing a molybdenum article requires that the metal be worked at high temperatures, e.g., brazing or welding can cause recrystallization because high temperatures exist locally at the brazing or welding site. The tendency of recrystallized molybdenum to become brittle is a deterrent to using molybdenum in many applications.